.\" -*- nroff -*-
.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" Copyright (c) 2020      Google, LLC. All rights reserved.
.\" $COPYRIGHT$
.TH MPI_Cart_sub 3 "#OMPI_DATE#" "#PACKAGE_VERSION#" "#PACKAGE_NAME#"
.SH NAME
\fBMPI_Cart_sub \fP \- Partitions a communicator into subgroups, which form lower-dimensional Cartesian subgrids.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Cart_sub(MPI_Comm \fIcomm\fP, const int\fI remain_dims\fP[], MPI_Comm\fI *comm_new\fP)

.fi
.SH Fortran Syntax
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_CART_SUB(\fICOMM, REMAIN_DIMS, COMM_NEW, IERROR\fP)
	INTEGER	\fICOMM, COMM_NEW, IERROR\fP
	LOGICAL	\fIREMAIN_DIMS\fP(*)

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_Cart_sub(\fIcomm\fP, \fIremain_dims\fP, \fInewcomm\fP, \fIierror\fP)
	TYPE(MPI_Comm), INTENT(IN) :: \fIcomm\fP
	LOGICAL, INTENT(IN) :: \fIremain_dims(*)\fP
	TYPE(MPI_Comm), INTENT(OUT) :: \fInewcomm\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
comm
Communicator with Cartesian structure (handle).
.TP 1i
remain_dims
The ith entry of remain_dims specifies whether the ith dimension is kept in the subgrid (true) or is dropped (false) (logical vector).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
comm_new
Communicator containing the subgrid that includes the calling process (handle).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
If a Cartesian topology has been created with MPI_Cart_create, the function  MPI_Cart_sub can be used to partition the communicator group into subgroups that form lower-dimensional Cartesian subgrids, and to build for each subgroup a communicator with the associated subgrid Cartesian topology. (This function is closely related to MPI_Comm_split.)
.sp
\fBExample:\fP Assume that  MPI_Cart_create( \&..., comm) has defined a (2 x 3 x 4)   grid. Let  remain_dims = (true, false, true). Then a call to
.sp
.nf
    MPI_Cart_sub(comm, remain_dims, comm_new)
.fi
.sp
will create three communicators, each with eight processes in a 2 x 4 Cartesian topology. If remain_dims = (false, false, true) then the call to MPI_Cart_sub(comm, remain_dims, comm_new) will create six nonoverlapping communicators, each with four processes, in a one-dimensional Cartesian topology.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
.sp
.nf
MPI_Cart_create
MPI_Comm_split